Weird persistent nightmare interaction.

Gun Bunny

I cast Rescue on a transformed persistent nightmare, and it returned to my hand, but since the flipped side has no casting cost, it couldn't gain the eight mana from Rescue. It stayed in my hand through the end of the turn, despite having a zero casting cost and an open creature slot on the battlefield.

The following turn, I matched enough Mana to charge the card in front of it, and partially charge the card behind it in my hand. The PN then cast, and hit the board as its transformed form. The following turn, it attacked, and bounced back to my hand naturally, and had full mana when it did.

I haven't tested this with other transform cards, but as none of them have a natural bounce mechanic, I can't say that the results would be similar.

BlessedOne

I tested this effect.
Persistent Nightmare is not recast after using Rescue. Instead, it goes to your hand as a no cost card.
The following turn, Persistent Nightmare dropped into play with no effect and costed no mana.
Once Persistent Nightmare did it's damage to the opponent, it went back into my hand as Startled Awake and had full mana.

Looks like the 8 mana went to Startled Awake (costs 6 mana to cast) behind the scenes with the 2 extra mana going into casting whatever else is in my hand.

I think this is a great corner case to generate mana... does not seem to be a glitch.

Gun Bunny

The weird part is that the card won't recast on the turn it's bounced to your hand, when it should.